This invention relates to a thermal cutout fuse which opens an electric connection between a pair of lead wires at a preset temperature.
Generally, the thermal cutout fuse of this type has heretofore been preferred to have a construction combining a thermally sensitive pellet with a mechanical spring on account of relatively satisfactory thermal properties and relatively high reliability of cutout motion. To be specific in this construction, a stationary contact and a movable contact are disposed within a housing and, while a mechanical spring constantly energizes the movable contact in the direction departing from the stationary contact, a thermal pellet which retains a solid state and occupies a fixed volume below a prescribed unsafe temperature (melting point) directly or indirectly represses the force of the spring tending to move the movable contact away from the stationary contact. In the normal state of the thermal cutout fuse (below the preset unsafe temperature), the intimate continuity of the two contacts is maintained and the electric continuity between a pair of lead wires connected to the contacts is similarly maintained. When the ambient temperature of the fuse rises past the preset unsafe temperature and the thermal pellet immediately melts and liquefies, the energizing force of the mechanical spring is effectuated to break the electric continuity between the two lead wires by causing the movable contact to be slid out in the direction of departing from the stationary contact.
In the conventional thermal cutout fuse of such a construction as described above, the movable contact is adapted to slide inside the housing perpendicularly to the surface thereof normally held in contact with the stationary contact. During this slide, the peripheral surface of the movable contact rubs against the inner wall surface of the housing. If the rubbing force thus generated differs, if very slightly, from one thermal cutout fuse to another to be manufactured, there may arise a possibility that the slide of the movable contact will be obstructed or the movable contact will be forced to assume a slanted posture during the slide. In any event, the conventional thermal cutout fuse has had a disadvantage that because of the rubbing during the slide, the movable contact will possibly fail to separate safely from the stationary contact. For the movable contact to produce safe slide, therefore, it becomes necessary to adopt a relatively large mechanical spring of high energizing force with due allowance or supplement the mechanical spring with an auxiliary spring adapted to keep the movable contact against the force tending to turn it aslant. Such special measures add to the complication, bulkiness, and production cost of the overall construction of the thermal cutout fuse.
With a view to improving the performance reliability of the conventional thermal cutout fuse, the present inventor developed a thermal cutout fuse having a novel cutout mechanism (U.S. Pat. No. 4,322,705).